Hey
I've climbed it twice, and will most likely do it again this year.
Here are a few quick tips:
* There's no actual climbing involved, it's just a hike. However, bringing your own hiking stick is highly recommended. Don't buy the heavy, shitty wooden sticks they sell at the 5th stations.
* Don't forget a flashlight, warm clothes (and I mean WARM) and rain gear.
* It's not prohibited in winter, but all mountain huts are closed, the trails must be covered with snow, and temperatures must be significantly lower. It's definitely possible, but I guess it gets closer to climbing than hiking and requires PROPER preparation...
* You don't need a guide.
* That one may sound obvious, but don't take the wrong trail when you descend.
* There are four trails starting from different altitudes, the required time to reach the summit greatly varies depending on which trail you choose.
* There's no real point in starting from the base unless you make a point of doing so. The "standard" way to do it: start from the 5th station in early afternoon (you'll have to adapt the time depending on which trail you choose), reach a mountain hut (the higher the better) around 7pm, rest for a couple hours, leave around 2:30am (again, adapt the time depending on the remaining distance), wait at the summit for the sunrise (around 5am).
* I don't know the exact regulations about camping, but last year I saw two guys setting up their tent next to a mountain hut.
* This has been said above, but avoid the weekends and national holidays at all cost. I climbed twice on a friday (last friday of august) and it wasn't crowded at all, especially last year but the weather was really average then.
* Mountain huts: reservation is mandatory, make sure you book a couple weeks ahead. It's 6000 yen per person and per night, each meal costs an additional 1000 yen. However, considering the quality and size of the "meals", I would recommend you bring your own bento.
* Altitude sickness: there's no way to know whether you'll suffer from it or not. Most people just have a headache but it can be much worse if you're unlucky. A friend of mine described it as a bad hangover, and I experienced it badly the first time. One thing you can do is take a one hour break when you arrive at the fifth station. And then climb SLOWLY.
Besides, I wrote a post on my blog about the hike, with more details and pictures of the subashiri trail, check it out, it's listed in the "Must read" section in the left column. It should give you an idea of what to expect.
Here are two good links with tons of information if you can read Japanese:
http://www001.upp.so-net.ne.jp/fujisan/
http://www.tozan.org/fuji/